Databricks·Software Engineer·Technical Phone Screen·Senior

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Apr 2026

Summary

Databricks software engineering interview that went deep into networking fundamentals. The main problem was a firewall-style IP filtering question that touched parsing, bit manipulation, and data structures all at once. More involved than I expected for a single coding round.

Questions Asked (1)

Given a list of CIDR blocks each labeled ALLOW or DENY, and an IP address, determine whether that IP is allowed or denied. The rules include precedence logic like most-specific match wins or DENY overrides ALLOW. You need to implement CIDR parsing, convert IPs to integers, and find matches efficiently using something like a bit-level trie. Also be ready to discuss how your approach changes for IPv6.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

This one took me a minute to even get oriented.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the precedence rules (most-specific match vs. DENY override) and edge cases like no match. Then outline a solution using a bit-level trie for efficient longest-prefix matching, and discuss how to extend it to IPv6 by generalizing to 128-bit addresses.

Pro tip: Mention that you would preprocess the rules into a trie for O(prefix length) lookup, and that you would handle IPv6 by using a unified 128-bit representation to avoid code duplication.

1. Clarify requirements and edge cases

Ask about precedence rules (most-specific vs. DENY override), default behavior when no rule matches, and whether rules can overlap. Confirm input formats for CIDR and IP.

2. Design data structures and algorithm

Propose converting IPs to integers and building a bit-level trie where each node represents a bit. Store the rule (ALLOW/DENY) at the node corresponding to the prefix length.

3. Implement lookup with precedence

Traverse the trie bit by bit, tracking the most specific matching rule. If DENY overrides, check for any DENY match along the path; otherwise, use the longest prefix match.

4. Extend to IPv6

Generalize the trie to 128 bits for IPv6, using the same logic. Discuss memory considerations and potential optimizations like path compression.

5. Analyze complexity and trade-offs

State time complexity O(W) where W is address width (32 or 128), and space O(N*W). Compare with alternatives like sorted lists or interval trees.

Key Points to Mention

CIDR parsing: splitting address and prefix length, validating input.
IP to integer conversion: using bit shifts for IPv4 and IPv6.
Bit-level trie: structure, insertion, and lookup for longest-prefix match.
Precedence handling: most-specific match wins vs. DENY override, and how to implement each.
IPv6 considerations: 128-bit addresses, memory usage, and potential optimizations.
Complexity analysis: time and space, and comparison with other data structures.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.